Various applications exist for the separation of gaseous mixtures, and in particular for the separation of nitrogen from atmospheric air to provide a source of highly concentrated oxygen. These applications include the provision of elevated concentrations of oxygen for (1) patients requiring the same in their breathing air and (2) flight personnel. Additional applications relate to processes such as drying high-purity gases such as separating hydrogen from hydrocarbons.
U.S. Pat. No. 2,944,627, issued Jul. 12, 1960, to Charles Skarstrom illustrates an early apparatus and method for fractionating gases having first and second fractionating vessels packed with molecular sieve material which selectively adsorbed one or more components of the gas so as to pass through an enriched product gas. A cross-over valving assembly allowed for a flow correspondence between the vessels and with a waste gas discharge. Product gas from a vessel was channeled to a primary product outlet with a large fraction being channeled to the other vessel. This fraction flushed the adsorbed or waste gases which had been trapped by the other vessel. The cross-over valve assembly cyclically switched the connection of the vessels with the incoming gas and the waste gas discharge. This cyclic switching of the vessels provided a regular flow of the primary product gas from the primary product outlet.
U.S. Pat. No. 3,313,091, to Berlin, improved upon the Skarstrom system through the utilization of a vacuum pump to draw adsorbed or waste gases from the vessel or bed being purged. Additionally this invention utilized a more complex valving system to produce a cycle which included vessel or bed pressure equalization, repressurization product production, bed pressure equalization, dumping, and purging.
U.S. Pat. No. 4,222,750, to Gauthier et al. related to a specifically defined timing cycle in which primary product gas from the adsorbing bed was passed through the desorbing bed during the desorption cycle. The vessels were connected to a compressor during a period of adsorption and to a vacuum pump during a period of desorption.
U.S. Pat. No. 4,449,990, to Tedford Jr. improved upon these prior art patents by teaching a method and apparatus for fractionating oxygen in which a pair of molecular sieve beds were cyclically connected in a timed cycle by a first cross-over valve (i.e., a four-way valve) with a source of pressurized air and a method of depressurizing the bed. The outlet ends of the beds were further connected by a flow path referred to as a pressure equalization flow path including a pressure equalization valve ("PE" valve) for selectively opening and closing the flow path. The path included two flow conduits including a limited conduit which is always open and a regulated flow conduit which has the PE valve for variable flow rate. Further in that patent, a timing and control circuit regulated the cross-over valve such that the pressure equalization valve was open 1 percent of the cycle duration before the cross-over valve reversed positions and was closed 2 percent of the cycle duration after the cross-over valve changed positions.
Generally in the prior art as represented by these and other patents, an equalization valve is disposed between a pair of check valves at the outlet ends of a pair of sieve beds in an oxygen concentrator system. While the equalization valve was referred to by Tedford as a pressure equalization valve (i.e., a "PE" valve), in this invention we will refer to the corresponding valve as a concentration equalization valve (i.e., a "CE" valve). Ultimately the same result is achieved of allowing a purge supply of product gas to enter a used bed; however, with a pressure-based supply, the rationale for using the valve varies slightly. Specifically the equalization valve acts to dampen the oscillation of the output gas concentration into the product tank which may otherwise occur. An oxygen concentration sensor measures and provides an indication of whether or not a certain oxygen level is met. For example, normal or acceptable operation may exhibit a green light at a reading of 85 percent or above; a yellow light may be illuminated at a reading between 73 and 85 percent; and a red light illuminates at a reading below 73 percent and the device subsequently powers down. This information is merely displayed to the patient or technician. That is, the technician manually controls the equalization valve in an effort to fine-tune the oxygen supply to the patient based on the indicator lights and oxygen readings.
In accordance with the present invention, the oxygen sensor communicates with the concentration equalization valve by means of the microprocessor which utilizes a closed-loop control to provide automated operation and optimization of oxygen levels from the sieve beds to the patient. In the prior art as represented by the '990 patent, the equalization valve is set manually. This valve provides for the cyclic flow of gas from the producing bed to the evacuated bed to provide sieve bed purge and to stabilize the oxygen content of the product gas passed into the product reservoir. Specifically, the valve settings change the time that the valve is open in one direction allowing purge gas (i.e., from one bed to the second) as compared to the time that the valve is open allowing flow in the second direction.
In the present invention, a closed-loop control circuit is provided to continuously and automatically regulate the setting of the concentration equalization valve. An oxygen sensor located between the reservoir or product tank and the patient, communicates information to the microprocessor which is programmed to evaluate the relative efficiencies of the sieve bed and thereby used to control operation of the concentration equalization valve.
Further in accordance with this invention, the operation of the oxygen concentrator is optimized through the use of information regarding the relative output flow rates between the sieve beds. The concentration equalization valve allows oxygen from the first sieve bed to mix with oxygen from the second sieve bed. The amount of time that gas is allowed to flow into the used sieve bed is determined by the concentration equalization valve adjustment. Closed-loop feedback with oxygen concentration provides the optimum setting for the concentration equalization valve (time allowed for balance of the first and second sieve beds). However, the optimum setting may change as a function of compressor flow output rate. This rate is dependent upon such things as altitude, compressor age, filter condition, and line voltage. By using a pressure transducer in the product tank, the pressure swing cycle can be controlled using electronic control means such as an integrated or remote microprocessor programmed with the appropriate software. The microprocessor is programmed to perform a step-wise adjustment in order to optimize the concentration equalization valve setting and consequently to optimize the oxygen concentration output.
The concentration equalization valve setting can be achieved initially by selecting the correct starting point. However, the relative efficiencies of the bed and the relative efficiencies of the corresponding concentration equalization valve setting of the beds may change during use. For example, after optimization at one extreme of the flow rate specification, a sudden change in flow rate may result in a concentration equalization valve setting inappropriate for the other extreme of the flow rate specification. The sudden change may cause the oxygen concentration to fall below the alarm threshold prior to achieving the new optimized concentration equalization valve setting causing the unit to shut down. By adjusting to a predetermined flow rate, the concentration equalization valve setting can be preprogrammed and the appropriate starting point also known. Therefore, the time required to achieve optimization is greatly reduced and further may be accomplished automatically, eliminating the need for an immediate service call by a technician.